I've spent the whole night polishing the new version (Rev.1) of the PCB.
I iterated a lot and finally, I only output one more signal that is tied to +3.3V when the input code is 0000. Cascading is easier and totally optional, the backplane must do its own circuit depending on the desired behaviour and cascade length.
The layout finally contains three mounting holes (3mm diameter) and several things have been moved around. You'll notice the crossed transistor symbol ;-)
Dimensions are 90×72mm. I have removed one via but 2 new ones have been required in the end.
The layout is VERY conservative. No silkscreen is needed, all the necessary information is available on the copper. For example the pad shape of the diodes are octogonal at the cathode end. The idea is to create the boards with old technology (even on bakelite :-D) so even the lousiest fab should work.
I'll have to figure out the orientation of the IV9 tube... There was a little issue with the footprint but nothing critical.
Anyway, the board is looking good®, it allows blanking-cascading and (I just realised) the single-relay input allows these relays to be used as hysteretic data storage...